1. Field of the Invention
The present invention relates to devices for evaluating the efficacy of steam sterilizers, and, more specifically, to devices that provide an indication of the presence of air in a sterilizer during the sterilization process.
2. Description of the Prior Art
Steam sterilization of an object is accomplished by placing steam in contact with the object. The object is cooler than the steam. Thus, the latent heat of the steam is transferred to the object. The latent heat of steam, which is 540 calories per gram, 100.degree. C. (212.degree. F.) at 14.7 psia and 525 calories per gram at 121.degree. C. (250.degree. F.) at 30 psia, makes steam particularly attractive as a sterilizing medium. For example, use of dry air at 320.degree. F. requires exposure for 60 minutes to achieve sterilization equivalent to that obtained by exposure for approximately 15 minutes to saturated steam at 250.degree. F.
To achieve effective steam sterilization, the steam should be fully saturated, that is, without the presence of contaminants such as air. If air is present within the steam, repeated collapse of the steam against the cooler surface of the object to be sterilized will occur, causing accumulation of air and the formation of an air pocket. Air pockets shield microorganisms on the object from the steam, and prevents transfer of a sufficient quantity of heat to the object, resulting in ineffective sterilization. Accordingly, prevacuum steam sterilizers remove air from the sterilization chamber prior to the introduction of pressurized vapor, or steam, into the chamber during the sterilization cycle. However, an ineffective prevacuum cycle can leave air in the chamber, which will remain there and accumulate during the sterilization cycle. Also, air can be introduced into the chamber as a result of equipment leaks and malfunctions, and through the steam supply.
To detect the presence of air during steam sterilization processes, Dr. J. Bowie and Mr. J. Dick described in 1963 a test pack that monitors prevacuum sterilization. The test pack consisted of 29 huckaback towels folded into fourths along their lengths and doubled across their widths to give eight thicknesses of cloth. Each folded towel was stacked, and steam sensitive autoclave tape, formed to resemble a St. Andrew's cross, was placed at various levels in the pack. The pack was placed within a dressing casket or overwrapped in fabric, and placed in the sterilization chamber. After the evacuation cycle had been conducted, steam was introduced into the chamber, where it entered the test pack and collapsed to water as it transferred its latent heat to the cooler towels. Any air that was present in the steam accumulated as the steam collapsed. The air was forced by the pressure within the sterilizer toward the density center of the test pack, where it formed pockets. Any air remaining in the test pack after the evacuation cycle also would be forced by the steam to the density center of the pack. The autoclave tape, through its steam indicator ink, detected the resulting air pockets. A uniform change in color of the steam sensitive indicator ink on the tape, from white to black, indicated completion of a successful sterilization cycle. However, an incomplete color change of the ink indicated an ineffective sterilization cycle. Accordingly, any air that migrated to the indicator ink located at the density centers of the test pack prevented steam from making contact with the entire indicator and, thus, prevented the ink from undergoing a uniform color change. The Bowie-Dick test is discussed further in The Lancet, Mar. 16, 1963, pp. 586-587. The Bowie-Dick protocol is further described in the National Standards and Recommended Practices for Sterilization: Recommended Practice--Good Hospital Practice--Steam Sterilization and Sterility Assurance, 6.7 (1988), published by the Association for the Advancement of Medical Instrumentation, 3330 Washington Boulevard, Arlington, Va. 22201.
The Bowie and Dick test pack performed several functions. First, and most basically, it separated steam from any air present in the steam, since the towels functioned like a heat sink, and included an indicator that indicated contact with steam. Importantly, it provided a "challenge" to the removal of air during the evacuation cycle. That is, it was more difficult to remove air from the test pack than it was to remove air from the objects that were to be sterilized, due to the natural entrapment of air by the interstices of the towels forming the test pack. Thus, the absence of air pockets from the test pack during sterilization justified the conclusion that air pockets were not present anywhere in the chamber during the sterilization process. Similarly, the test pack provided a "challenge" to the entry of steam into the test pack during the sterilization cycle, justifying the conclusion that steam had adequately contacted the articles undergoing sterilization if the indicator ink on the tape in the pack changed color uniformly. Finally, the towels of the test pack absorbed the water created by the collapsing steam, thus preventing it from travelling to the autoclave tape where it could revaporize and contribute to a false indication. In summary, then, the Bowie and Dick test pack offered (i) a challenge to removal of air from the test pack, (ii) a heat sink that absorbed the latent heat of steam and separated air and steam, (iii) resistance to penetration by steam into the pack, (iv) an indicator that detected the presence of air and (v) an absorbent that retained the water generated by the change of state of the steam.
Nonetheless, Bowie and Dick type testing proved less than satisfactory due to the need for cotton towels, varying directions by manufacturers in the use of their indicator sheets, variations in folding techniques, non-uniform towel conditions, variations in the outer wrapping material utilized, and the presence of laundry chemicals on the towel, which could react with the indicator. Therefore, workers in the art attempted to develop devices for indicating the presence of air during sterilization that afforded more uniformity in use, and more reliable results, and that provided the benefits of the Bowie-Dick test pack.
Joslyn U.S. Pat. No. 4,115,068 discloses an air indicating device for use in a steam or gas sterilizer comprising an upright, insulated tube with a closed top and open bottom. The upper portion of the tube contains a thermal indicator surrounded by a heat sink material. Steam enters the tube through the bottom and is forced toward the top of the tube. The steam that contacts the heat sink will be condensed; however, the steam that contacts the indicator strip will transfer its heat and humidity to the strip. As the temperature of the heat sink rises during the sterilization cycle, it can revaporize the condensed steam, which will provide a false indication of the presence of saturated steam. Additionally, the tube does not offer a challenge to air removal during the evacuation cycle, and it does not offer resistance to steam penetration during the sterilization cycle.
Augurt U.S. Pat. No. 4,486,387 discloses a disposable test pack for determining the effectiveness of a prevacuum steam sterilization cycle. The pack consists of a steam indicator sheet placed between stacks of porous paper sheets. The pack is intended to simulate a Bowie-Dick test pack. However, the evacuation of air from and introduction of steam into the stack will occur on all six sides of the test pack, and will tend to follow the path of least resistance. Further, the stack of porous sheets consists of an inner "core" of sheets and an outer "shell" of sheets. The sheets constituting the "core" are of a porosity that is greater than those constituting the "shell." Therefore, air and steam travel will occur to a greater extent through the inner "core," and the desired effect of the two different porosities is eliminated. Also, depending on the smoothness and degree of contact among the sheets, within each layer, the flow of steam into the stack will vary, and, correspondingly, the indicator will give varying results. Finally, the degree of dryness of the paper forming the layers will affect the indication provided by the stack.
Dyke and Oshlag U.S. Pat. No. 4,594,223 discloses an insulated heat sink placed in "series" with a chamber containing an indicator for detecting the presence of air within a steam sterilizer. The heat sink can be fibrous or a metal coated with an insulator. As steam contacts the heat sink it gives up its latent heat, which causes the steam to change state, and accumulate any air mixed with the steam. Newer prevacuum steam sterilizers use a conditioning-evacuation cycle prior to sterilization, which replaces the original deep or continuous evacuation. This newer cycle typically commences with a one minute steam purge with the drain open followed by closing of the drain a vacuum pulse and three intermittent steam and vacuum pulses. Each pulse occurs at a prescribed pressure to condition the load and eliminate air prior to sterilization. During the steam purge and pulses, the temperature of the heat sink rises, and its ability to function is compromised. Therefore, the heat sink cannot effectively separate air from steam during the critical sterilization cycle.
Accordingly, despite the efforts of workers in the art, there remains a need for an effective indicator of the presence of air during steam sterilization that provides the benefits of a Bowie-Dick test pack.